node, node-template: disable GRANDPA observer (#5011)

* node-template: disable grandpa observer

* node: disable grandpa observer

* node: add doc about grandpa-voter infallible task

* grandpa: remove grandpa observer from public API

* grandpa: ignore observer_enabled field in config

substrate/client/finality-grandpa/src/lib.rs

@@ -96,7 +96,6 @@ mod voting_rule;
 pub use finality_proof::FinalityProofProvider;
 pub use justification::GrandpaJustification;
 pub use light_import::light_block_import;
-pub use observer::run_grandpa_observer;
 pub use voting_rule::{
 	BeforeBestBlockBy, ThreeQuartersOfTheUnfinalizedChain, VotingRule, VotingRulesBuilder
 };
@@ -551,7 +550,7 @@ pub fn run_grandpa_voter<B, E, Block: BlockT, N, RA, SC, VR, X>(
 	Client<B, E, Block, RA>: AuxStore,
 {
 	let GrandpaParams {
-		config,
+		mut config,
 		link,
 		network,
 		inherent_data_providers,
@@ -560,6 +559,12 @@ pub fn run_grandpa_voter<B, E, Block: BlockT, N, RA, SC, VR, X>(
 		voting_rule,
 	} = grandpa_params;
 
+	// NOTE: we have recently removed `run_grandpa_observer` from the public
+	// API, I felt it is easier to just ignore this field rather than removing
+	// it from the config temporarily. This should be removed after #5013 is
+	// fixed and we re-add the observer to the public API.
+	config.observer_enabled = false;
+
 	let LinkHalf {
 		client,
 		select_chain,

substrate/client/finality-grandpa/src/observer.rs

@@ -150,6 +150,9 @@ fn grandpa_observer<B, E, Block: BlockT, RA, S, F>(
 /// listening for and validating GRANDPA commits instead of following the full
 /// protocol. Provide configuration and a link to a block import worker that has
 /// already been instantiated with `block_import`.
+/// NOTE: this is currently not part of the crate's public API since we don't consider
+/// it stable enough to use on a live network.
+#[allow(unused)]
 pub fn run_grandpa_observer<B, E, Block: BlockT, N, RA, SC>(
 	config: Config,
 	link: LinkHalf<B, E, Block, RA, SC>,

substrate/client/finality-grandpa/src/tests.rs

@@ -1376,7 +1376,7 @@ fn finalize_3_voters_1_light_observer() {
 
 	run_to_completion_with(&mut runtime, 20, net.clone(), authorities, |executor| {
 		executor.spawn(
-			run_grandpa_observer(
+			observer::run_grandpa_observer(
 				Config {
 					gossip_duration: TEST_GOSSIP_DURATION,
 					justification_period: 32,